This is a continuation of International PCT Application No. PCT/JP99/06643 filed Nov. 29, 1999.
The present invention in general relates to an inverter control apparatus used in air conditioners. More particularly, this invention relates to an inverter control apparatus for controlling a compressor motor.
The air conditioner comprises an indoor unit (placed inside a house) and an outdoor unit (placed outside the house) Conventionally, in the outdoor unit of the air conditioner, there has been provided an inverter control apparatus for controlling a compressor motor (an induction motor, a DC brushless motor, etc. ) that is driven by an output AC voltage. The output AC voltage is obtained as follows. A converter and a smoothing capacitor once converts a commercial AC voltage into a DC voltage, and the inverter again converts the DC voltage into the output AC voltage by a PWM (Pulse Width Modulation) control.
The conventional converter is provided with a diode bridge circuit, and this converter converts a commercial AC voltage into a DC voltage. The smoothing capacitor is connected in parallel to the converter, and smoothes the DC voltage that has been obtained by the conversion of the converter. The inverter is composed of a switching element like a witching transistor. This inverter is a three-phase inverter that converts the DC voltage into an output AC voltage of a three-phase AC having a predetermined frequency by the PWM control for ON/OFF controlling the switching element at a predetermined timing. This inverter is input with a PWM switching pattern for instructing a switching timing of the switching element.
This PWM switching pattern is generated based on a target voltage corresponding to the operation frequency of the compressor motor by a known V/F (Voltage/Frequency) control. The operation frequency takes a value according to an air-conditioning load. For example, the operation frequency takes a large value when there is a large difference between a set temperature of the air conditioner and a room temperature.
The output AC voltage from the inverter is expressed as a surface area (i.e. surface area S) that is a product of a DC voltage VDC and a PWM width W as shown in FIG. 7. In the PWM control, the PWM width W is controlled so that the output AC voltage coincides with the target voltage. The DC voltage VDC shown in this drawing is a DC voltage that has been smoothed by the smoothing capacitor, and the PWM width W corresponds to a period during which the switching element of the inverter is ON.
When the commercial AC voltage is supplied to the converter, the converter full-wave rectifies this commercial AC voltage to convert this voltage into the DC voltage. Then, the smoothing capacitor smoothes this DC voltage, and supplies the smoothed DC voltage to the inverter.
In this case, the PWM width W is calculated from the DC voltage VDC and the target voltage (i.e. the surface area S) shown in FIG. 7. In other words, the PWM width W is a result of dividing the target voltage by the DC voltage VDC. In this calculation of the PWM width W, the DC voltage VDC is handled as a constant value. A PWM switching pattern corresponding to the PWM width W is input to the inverter.
Based on the above arrangement, the inverter ON/OFF controls the switching element at a predetermined timing according to the PWM switching pattern, thereby to convert the DC voltage into the output AC voltage having a predetermined PWM width. This output AC voltage is supplied to the compressor motor so that the compressor motor is driven.
As mentioned above, in the conventional inverter control apparatus, the DC voltage VDC is handled as a constant value for calculating the PWM width shown in FIG. 7. However, in reality, the commercial AC voltage varies, therefore, the DC voltage VDC also varies.
Therefore, according to the conventional inverter control apparatus, there arises a difference between the PWM width W calculated and a theoretical value for carrying out an optimum control, when the DC voltage VDC has varied. In other words, the PWM width is calculated as a constant value regardless of a variation in the DC voltage VDC shown in FIG. 7 following the variation in the commercial AC voltage. As a result, there arises a situation that the output AC voltage (corresponding to the area S) actually supplied from the inverter to the compressor motor cannot follow the target voltage.
FIGS. 8(A) and (B) show a case where an output AC voltage VOUTxe2x80x2 from the inverter varies following the variation in the DC voltage VDC. FIG. 8(A) shows a state that although it is desirable that the DC voltage VDC takes a constant value, the DC voltage VDC increases along lapse of time and then decreases, due to the influence of the variation in the commercial AC voltage. When the DC voltage VDC has varied like this, an output AC average voltage VAOUTxe2x80x2 that is a time-averaged output AC voltage VOUTxe2x80x2 also varies as shown in FIG. 8(B).
As explained above, according to the conventional inverter control apparatus, the PWM width W is calculated based on the DC voltage VDC as a constant value, despite the fact that the VDC varies every moment from a DC voltage VDC1 to a DC voltage VDC2, . . . , and to a DC voltage VDC5, as shown in the drawing. Therefore, surface areas S1 to S5 of output AC voltage elements V1xe2x80x2 to V5xe2x80x2 also take different values respectively.
As a result, the conventional inverter control apparatus has had the following problem. When the DC voltage VDC has varied following the variation in the commercial AC voltage, the output AC voltage VOUTxe2x80x2 that is supplied from the inverter to the compressor motor is deviated from the target voltage. As a result, it has not been possible to carry out an optimum operation of the compressor motor.
Particularly, when the commercial AC voltage has decreased suddenly, the output AC voltage VOUTxe2x80x2 becomes less than a minimum rated voltage of the compressor motor, which is a voltage shortage state. This results in an occurrence of a stalling. On the other hand, when the commercial AC voltage has increased suddenly, the output AC voltage VOUTxe2x80x2 exceeds a maximum rated voltage of the compressor motor, which is an overvoltage state. This results in a flow of an excess current to operate the protection circuit, and stops the operation of the compressor motor (a stop due to an overcurrent).
The power source situations (rated values, and stability, etc. of a commercial AC voltage,) in the world are different between the countries (regions). Therefore, in countries where the stability of the commercial AC voltage is low, the use of the conventional inverter control apparatus can easily invite the occurrence of the above-described voltage shortage and overvoltage. Therefore, the risk of a frequent occurrence of the stalling and a stop due to an overcurrent becomes very high. In other words, according to the conventional inverter control apparatus, there has been a problem that the stability of the control of the compressor motor is easily controlled by the power source situation.
A DC current IDC shown in FIG. 9(A) includes a ripple IR1, as the inverter control apparatus uses a low-cost circuit for reducing the cost of. This DC current IDC is a voltage that has been smoothed by the smoothing capacitor. The size of the ripple IR1 is determined by a circuit constant and the load.
According to the conventional inverter control apparatus, the DC current IDC that includes the ripple IR1 shown in FIG. 9(A) is switched by the PWM control. Therefore, an output AC current IOUTxe2x80x2 from the inverter shown in FIG. 9(B) also includes a ripple IR2xe2x80x2. A peak value of this ripple IR2xe2x80x2 corresponds to a peak value of the ripple IR1 (reference FIG. 9(A).
From the above, the conventional inverter control apparatus has had also the following problems. It is necessary to use an overcurrent protection circuit that breaks a DC when the DC flowing through the switching element of the inverter exceeds a threshold value, and to use a switching element that has a large capacity and high precision. This has been expensive.
It is an object of the present invention to provide an inverter control apparatus capable of operating an AC load at low cost and in an optimum state, and also capable of operating the AC load in an optimum state under any power source situation.
The inverter control apparatus according to this invention comprises a converting unit which converts a commercial AC voltage into a DC voltage; an inverting unit which converts the DC voltage into an output AC voltage of a predetermined frequency by a pulse-width modulation system based on an assigned pulse width, and supplies the output AC voltage to an AC load; an instantaneous DC voltage detecting unit which detects an instantaneous value of the DC voltage; and a correcting unit which corrects the pulse width so as to maintain the output AC voltage at a desired value following the variation in a result of a detection by the instantaneous DC voltage detecting unit.
Thus, in the inverter control apparatus of this invention, the instantaneous value of a DC voltage also varies following the variation in a commercial AC voltage. This variation in the instantaneous value of the DC voltage is also reflected in a result of a detection carried out by an instantaneous DC voltage detecting unit. Then, the correcting unit corrects the pulse width following the variation in the detection result. As a result, the output AC voltage supplied from the inverting unit to the AC load is maintained at a desired value without receiving an influence of the variation in the instantaneous value of the DC voltage following the variation in the commercial AC voltage.
In other words, it is possible to supply an output AC voltage of a desired value to the AC load without receiving an influence of the variation in the commercial AC voltage. As a result, it is possible to operate the AC load in an optimum state of load characteristics and efficiency.
Further, it is preferable that the correcting unit corrects the pulse width based on a result of multiplying a ratio of a preset reference voltage to a detection result of the instantaneous DC voltage detecting unit, and a target voltage as a target value of the output AC voltage.
Thus, the correcting unit corrects the pulse width based on a result of multiplying a ratio of a reference voltage to a detection result of the instantaneous DC voltage detecting unit, and a target voltage. Therefore, it is possible to supply an output AC voltage of a desired value to the AC load without receiving an influence of the variation in the commercial AC voltage. As a result, it is possible to operate the AC load in an optimum state of load characteristics and efficiency.
Further, the inverter control apparatus may further comprise a setting changing unit which changes a setting of the reference voltage according to the commercial AC voltage.
Thus, the setting changing unit changes the setting of the reference voltage according to the commercial AC voltage. Therefore, it is possible to supply an output AC voltage of a desired value to the AC load in countries and regions where the power source situations (rated values, and stability, etc. of a commercial AC voltage,) are different. As a result, it is possible to operate the AC load in an optimum state regardless of countries and regions.
Further, it is preferable that the correcting unit corrects the pulse width by calculating the ratio for every one period of a modulation carrier in the pulse-width modulation system.
Thus, the pulse width is corrected for everyone period of a modulation carrier. Therefore, as the influence of the ripple included in the commercial AC voltage is reduced, it is possible to avoid a current breaking due to an overcurrent attributable to the ripple, and it is also possible to increase the reliability. Further, it is possible to lower the cost (an overcurrent breaking circuit) for avoiding the ripple.
Further, it is preferable that the correcting unit corrects the pulse width by calculating the ratio for every n periods (=equal to or above 2) of a modulation carrier in the pulse-width modulation system.
Thus, the ratio (a reference voltage/a detection result of the instantaneous DC voltage detecting unit) is calculated for every n periods of a modulation carrier. Therefore, it is possible to decrease the number of calculating the ratio per unit time, as compared with the case of calculating the ratio for every one period. As a result, it is possible to lower the cost as the correction can be realized by a low-cost arithmetic and logic unit.
Further, it is preferable that the instantaneous DC voltage detecting unit directly detects the DC voltage by a non-insulating circuit that has been grounded to be in a potential common to that of the correcting unit.
Accordingly, it is possible to shorten the detection time, as compared with the case of detecting a DC voltage using an insulating circuit like a photo-coupler or the like.